UV rays having a wavelength between 200 and 380 nm are known to be harmful to the human eye. In particular, they can accelerate ocular ageing which can lead to an early cataract or to more extreme disorders such as photokeratitis or “snow blindness”, These damages can be prevented by incorporating UV-absorbers in ophthalmic lenses.
Three different methods can be used to prepare UV-absorbing ophthalmic lenses. The first method is the impregnation of a polymerized lens in a bath containing a UV-absorber as disclosed in European patent No 1 085 349. However, this method adds a step to the production process of the lens, which is not desirable in terms of cost and time.
The second method is the coating of a substance capable of absorbing UV rays onto the surface of ophthalmic lenses as disclosed in U.S. Pat. No. 5,949,518. However, the incorporation of high amounts of UV-absorbers in a coating weakens its mechanical properties.
The third method is the incorporation of a UV-absorber in the bulk liquid formulation (i.e. before polymerization) as taught in European patent No 1 085 348. In this document, a thermosetting composition comprising a diethylene glycol bisallylcarbonate or bis(β-epithiopropyl)sulfide monomer, diisopropyl peroxydicarbonate as a catalyst and 2-(2-hydroxy-4-octyloxyphenyl)-benzotriazole as a UV-absorber is cast into a mold for lenses and heated until polymerized. However, this method is unable to provide a lens that can both efficiently absorb UV rays and that does not exhibit undesirable yellowing.
Yellowing of the lens, as measured by the yellow index, can be caused by two degradation mechanisms:                during polymerization, interaction between radicals and UV absorber generates a yellowing effect which is particularly apparent when high concentrations of catalysts are used to initiate polymerization, especially with monomers having weak reactivity such as allyl monomers;        during ageing of the lens, UV rays interact with the polymer matrix and UV absorbers, generating a yellowing effect.        
Yellowing of the lens could be prevented if less catalyst were used (which may happen if an additive or impurity reacts quickly with catalyst) but polymerization would not be complete and mechanical properties of lenses would not be acceptable.
European patent application No 2 172 792 mentions that the yellow index of a lens made with a thermosetting composition comprising a UV absorber can be reduced by introducing specific dyes or pigments, namely a cobalt oxide and alumina compound and a nanodispersion of gold particles. However, the preparation process of the lens is complex and the lens obtained with this composition has decreased transmittance or a greyish shade.
There is thus a need for a thermosetting composition that can produce ophthalmic lenses that have a UV cut of 380 nm and a yellow index that is less than 1.5 while at the same time having sufficient transparency.
The Applicant has found that this need could be met by adding a UV absorber and a specific anti-yellowing additive in the thermosetting composition. This specific anti-yellowing additive presents two chemical functions:                Anti-yellowing function. Surprisingly, sulfide or disulfide containing compound, dithiane or trithiane derivatives yield good results. Usually, in the ophthalmic industry, sulphur-containing compounds are known to bring yellow colour to lenses. In this invention though, sulphur compounds provide for less yellow lenses. Piperidine derivatives were also identified as very efficient anti-yellowing additives.        Solubilisation function. A good solubilisation of anti-yellowing additives in monomers is required to avoid defects such as haze or colour unhomogeneity. In allylic monomers used in ophthalmic industry, solubilisation is facilitated when polar groups such as (thio)esters, (thio)carbonates, (thio)carbamates or (thio)amides are present. The number of such polar groups in relationship with the molecular weight of these compounds defines good solubility parameters.        